YC tells founders that one of the fastest ways to kill your company is to base your product on code that's not legitimate to use (i.e., that you didn't write yourself or that is used in breach of its license). That's because it's one of the fastest ways to kill funding rounds, acquisitions and enterprise deals. Not everyone listens or understands.

It even asks (or at least it did the last time I checked) in the application form, if you wrote your code yourself, to raise the issue of IP ownership/licensing from the start.